Ink-jet printers employ a reservoir of ink to supply a printhead. In thermal ink-jet printers, the printhead comprises a plurality of nozzles in a nozzle plate. Underneath each nozzle is a firing chamber, commonly fed with ink from a plenum connected to the reservoir. Ink is expelled through the nozzles by a resistor element in the chamber, which is energized in response to a signal from a microprocessor in the printer.
In such ink-jet printers, the reservoir and printhead are contained on a replacable unit referred to as a cartridge or pen. The reservoir portion itself is a foam, which has been loaded with ink. Negative pressure at the printhead ensures a continual supply of ink, so long as there is ink in the reservoir.
In the past, it has been practically impossible to detect the ink fluid level in foam-containing cartridges, or pens, due to the gradual change in appearance between the filled and the drained semi-wet portions of the foam. Yet, it would be helpful to the user of the printer to be able to readily ascertain whether there is sufficient ink. This is particularly true if a large print job is scheduled, since it is obviously a waste of time and ink to print part of a job and run out of ink during the printing thereof.